Macroevolutionor - the evolution of species

The Biological Species Concept: A species is a population or group of populations whose members have the potential to interbreed in nature to produce fertile offspring, and whose members are reproductively isolated from other such groups.

But this is a hellish definition to test species…

Darwin (1853): “ After describing a set of forms as distinct

species; tearing up my [manuscript], & making them one species; tearing that up & making themseparate, & then making them one again (which has happened to me) I have gnashed my teeth, cursed species, & asked what sin I committed to be sopunished.”

(Darwin was trying to separate barnacles.)

More practical problems:

1) How do we deal with asexual species (bacteria, many fungi, some fish, and assorted others)? They do not interbreed. They don’t fit the definition.

2) How do we deal with fossils? Are morphological differences sufficient to define species? That would not always work with living species.

3) Does a different morphology necessarily mean a different species? No!

4) What about geographically separate populations? How can we tell if they can interbreed “in nature”? The species concept may answer this question.

What happens when apparently different species arebrought together artificially (in zoos, etc.)?

Lions and tigers interbreed in zoos, producing ligers.

Bison and cattle breed on farms, producing commercially valuable ‘beefalos’. It only works successfully in one direction: bull with buffalo cow

Why?

Note the front shoulders of this beefalo. A femalecow can’t handle birthing the calf of the cross withoutdangerous distress.

So, how do new species evolve?

Through accumulation of genetic differences.

Eventually, sufficient difference has accumulated toreproductively isolate the two groups, even if theyshould be in contact.

Those differences usually accumulate while thegroups (populations) are geographically isolated.

An example from northern Canada: There are two major species of lemmings (collared lemming, brown lemming). Lemmings lived in the shadow of receding glaciers. As James Bay opened with glacial melting, the western and eastern populations of lemmings were separated (geographically isolated). Differences accumulated over time producing the two lemming species we have today.

However, there are other mechanisms that canproduce reproductive isolation even while groupsare in close proximity. These barriers are separatedinto pre-zygotic (preventing zygote formation) andpost-zygotic (affecting embryo development).

Pre-zygotic barriers:

1) Habitat isolation - e.g., marsh versus forestwarblers, or parasites limited to different hostspecies

2) Temporal isolation - e.g., breeding seasons inyellow-headed (May - early June) and red-winged(late June - early July) blackbirds.

3) Behavioural isolation - it’s the species specificmating dances that many of the 400 HawaiianDrosophila do in courtship that reproductivelyisolates them.

4) Mechanical isolation - e.g. imagine Great Danesand chihuahuas attempting to mate

5) Gametic isolation - sperm (or pollen) and eggmust be chemically compatible. In plants, pollendoesn’t germinate, pollen tubes fail to grow,… Inanimals, membrane proteins don’t match, andfertilization doesn’t occur

post-zygotic barriers

1) hybrid inviability - genetically programmed development is a complicated process. Speciesdiffer in this program. When genomes are mixedin a hybrid, conflicts result in the embryo failingto develop completely. They generally don’tsurvive.

2) hybrid sterility - the hybrids survive to maturity,but cannot produce viable offspring. The reasonis usually traceable to incompatible chromosomesthat don’t match up in meiosis. An example: horses and donkeys mate, the offspring (mules)are viable, but sterile

3) hybrid breakdown - first generation hybrids areviable and fertile, but the second generation (orbeyond) are feeble (low survivorship and greatlyreduced reproductive output) or sterile. Thisoccurs among different species of cotton.

In the absence of successful barriers to hybridization - gene exchange between ‘species’ occurs. It is called introgression. With gene flow, new species can’t form. So, think of populations in the process of accumulating differences never being able to accumulate sufficient differences to speciate.

Even though various mechanisms can lead toreproductive isolation, the most common remainsgeographic isolation. The separation can occur atthree ‘levels’:

1. Allopatry - allopatric speciation

2. Parapatry - parapatric speciation

3. Sympatry - sympatric speciation

Allopatric speciation

- A population becomes geographically fragmented A body of water (river, ocean) may separate them

(e.g. the lemmings). A small group may colonize an island (e.g. Darwin’s finches Plate tectonics may cause the rise of mountains

between them.

- Either due to environmental differences betweensites (differences in regimes of natural selection)or chance events in small colonist groups (drift)genetic differences between groups accumulate.

- Eventually, sufficient differences accumulate to prevent interbreeding. At this point we say a new species has evolved.

- Differences appear and spread more rapidly in small populations (drift!; mutation is not more likely in small populations)

-Frequently, it is marginal populations within what had been a large, widespread population that become isolated (more likely to encounter different environments).

- Adaptive radiation may occur as small groups become repeatedly isolated, e.g. Darwin’s finches or Hawaiian silverswords.

A ground finchSharp-beaked finch (Geospiza nebulosa)

A cactus finchSmall cactus finch (Geospiza scandens)

Another ground finchwith a smaller billSmall ground finch (G. fuliginosa)

A survey of the full set of Darwin’s finches

A few of the wild and wonderful Hawaiian silver-swords. All grow at upper elevations in Hawaii. Themost remarkable is the Haleakala silversword, whichgrows in the cone of an ‘active’ volcano on Maui.

Haleakala silversword,Argyroxiphium sandwichense

Island adaptive radiation can go in strange directions. These are ‘sunflowers’ that result from adaptive radia-tion on the island of St.Helena in the south Atlantic.

Parapatric speciation - Populations are not separated; their boundaries contiguous

- Speciation can occur when a strong environmental gradient extends across the boundary between populations

- Differences in selection pressures must be great enough to overwhelm any gene flow across the boundary

- Two examples: distribution over a mountainside distribution over a mine spoil

gradient

Imagine a mountainside. It gets colder asyou climb. Temperature changes by 10°Cper kilometer. One species lives at the bottom of the slope, the other at the top.Differences in temperature adaptationsmay mean that reproductive success isvery much lower in the ‘wrong’ part of theslope. Slowly, populations come to differin many ways, and parapatric speciationoccurs.

The other example is of grasses growing on and offland of a mine in Wales that produced lead. Heavymetals (lead, copper, nickel) are poisonous to many(most) plants. Selection on mine lands produced a variety of Agrostis tenuis that was tolerant of lead. A part of the change (mutation) giving tolerance was a shift in flowering time. Thus, although tolerant andintolerant plants grow in adjacent areas, there islittle or no gene flow between populations, andspeciation can occur.

Sympatric speciation

- Populations overlap in distribution (sym - same; patra - country). Then how can they become reproductively isolated?

- Two accepted ways: by host specialization by becoming polyploid

- Host specialization - when host is both feeding and mating site, a change in host can isolate the shifted population. Example in text:

Rhagoletis pomonella normally feeds and mates on hawthorn fruits. Some switched in NY in 1864 to feeding on apples. In 1960 some switched again, to cherries.

- becoming polyploid:autopolyploids - double chromosome numberby non-disjunction or nuclear fusion in meiosis.Diploid gametes self-fertilize. Result istetraploid. It cannot backcross with parents, butis fertile with a like type.

Allopolyploidy - fertilization involving gametes fromtwo different species. Interspecific hybrids areusually inviable or sterile (due to failure of chromosome pairing in synapsis of prophase inmeiosis; chromosomes aren’t really homologues),but… Non-disjunction in the first generation of the hybrid can make a viable, fertile hybrid.

Is allopolyploidy important?

Did anyone have bread (pizza, big Mac, sandwich)for lunch? The bread was made from wheat.

Allopolyploidy produced Triticum aestivum, or bread wheat. Its chromosome complement is 2n=42.That complement arose by spontaneous hybridizationof 2 other wheat grasses with 28 and 14 chromosomes.

2n=28 --> n=14 in gametes2n=14 --> n=7

seems incompatible, but by non-disjunction in meiosisof this hybrid, a fertile species with 2n=42 was formed.

Adaptive radiation occurs on continents, as well, but continental drift and plate tectonics brings faunas into contact, and there are then extinctions. Here are drawings of South American mammals driven extinct after the rise of Central America permitted exchange. Only the armadillo and opossum successfully moved north. A host of larger North American mammals crossed southward, and drove ecologically similar species extinct. The text has a diagram of the North American mammals that were (at least temporarily) successful in the south.

Here are diagrams of what they drove extinct:

Large, mastodon-like

Probably related to condylarths, with camel-like habits

Extinction of species is as important to what weobserve today as is speciation. There is a nominalbackground rate of species disappearance (extinction).

However, in the history of life on earth there have beenperiods when much larger numbers of extinctionsoccurred. These are called mass extinctions. Therehave been 5 mass extinctions (and humans are almostcertainly driving a 6th.

Causes of some are not known. One seems relativelywell explained - the mass extinction that occurred 65 MYBP, and eliminated dinosaurs, making room fordiversification and enlargement of the mammals.

The Cretaceous mass extinction was caused by acombination of climate change and the collision ofa ~17km diameter asteroid into the Yucatan peninsula.

The effects of collision were much like the nuclearwinter that would be caused by global nuclear warfare:

Intense fires over much of Mexico and the U.S., a global dust cloud darkening the skies for months, asudden change in climate as a result, death of plants,and therefore a lack of food to support the hugevegetarian dinosaurs, leading to the death of the meateaters.

We are driving another mass extinction. Manyspecies are going extinct each day, though we don’teven know their names, and may not have evendiscovered them yet. How?

We are cutting down tropical forests in South America,Africa, and Asia for lumber, firewood, and conversionto agricultural land, both on the large scale and as aresult of population increase and ancient practice ofslash and burn farming.

The effect of this is not only loss of plant species, butloss of the diversity of insect and other species directlyor indirectly dependent on the plants.

Tempo and mode in EvolutionThere are two views of the rate of apparent change inspecies:

1) the microevolutionary view - new species formation results from gradual accumulation of phenotypic (usually seen as morphological) change.

2) the punctuated equilibrium - most of the morpho- logical change becomes apparent when species initially form. Populations are then very small. Selection can rapidly move the characteristics of the entire species, and drift can lead to rapid change. Through the remainder of the species’ history, there is little evident change.

The result is a history in the fossil record of longstasis (equilibrium, constancy) punctuated by shortperiods of dramatic change.

Is only one of these hypotheses correct, and the otherwrong? No!

A punctuated equilibrium is evident in the fossilrecord. The ‘sudden’ change may represent 1000s ofyears of gradual change, but it looks rapid when viewed on a geological time scale.